1. Field of the Invention
The present invention relates to power supply equipment. More specifically, the invention relates to a power supply apparatus and a power supply method that use a battery and a capacitor to deliver a stable power output. The power supply apparatus and method are capable of saving energy via reciprocal switches of the polarity connection between the battery and the capacitor.
2. Description of the Related Art
Batteries have become a necessity in modern life, and are used daily in various areas from automobiles to consumer products such as cell phones, laptops and music players. Batteries depend on chemical reactions for energy conversion at charging and discharging, and are normally designed for applications using low powers. Because the chemical reactions of batteries require overcoming some energy barriers, batteries are prevented from fast charging and discharging. Though lead acid batteries are well known by high power densities as they are commonly used to start automobiles, the batteries have short service time for delivering such high currents. Theoretically, all batteries can be imparted a high power density with breakthrough in their materials. In return, the use time and lifetime of the batteries are compromised, and the devices tend to be bulky if they are made to work long.
In comparison, supercapacitors utilize rapid surface adsorption and desorption for energy conversion. When the electrodes of the supercapacitors are energized at charging, ions of the electrolyte enclosed in the devices will be quickly adsorbed at the interface of electrodes and electrolyte. The ions accumulated count for the capacitance of capacitors, or energy stored in the capacitors. When supercapacitors are controlled to discharge, desorption of ions can proceed quickly as well. Hence, supercapacitors have much higher power densities than batteries.
Supercapacitor is also known as ultracapacitor and electric double-layer capacitor. Activated carbon is the most popular adsorptive material for fabricating the supercapacitor. Due to the large surface area of activated carbon, supercapacitors can store several order of magnitude of energy higher than that of conventional capacitors, for example, aluminum electrolytic capacitors. For the convenience of manufacturing, both electrodes of a supercapacitor are frequently made of the same activated carbon in the same formulation and the same preparation process. By design, the two electrodes of a supercapacitor are symmetrical carrying no polarity until the supercapacitor is charged. On the contrary, batteries and conventional capacitors have designated anode and cathode made of different materials. In terms of polarity, the two electrodes of a battery are not interchangeable.
It is when a supercapacitor is connected to a power source for charging that the polarity of its two electrodes is decided. The electrode hooked to the positive pole of the power source will be positively charged and the other electrode negatively charged, indicating that the polarity of the electrodes of a supercapacitor is created through charging. Once a charged supercapacitor releases its stored energy to a load completely, its electrodes resume the non-polar state. In the next charging stage, either electrode, regardless of its polarity induced in the previous charging stage, of the supercapacitor can be connected to the positive or negative pole. The forgoing switch of electrodes for charging causes no damage to the supercapacitors for the electrodes are symmetrical with the same chemical identity. Such switching of polarity connection is not permitted for batteries or conventional capacitors for the polarity of their electrodes are fixed. If the electrodes of the latter are misconnected, some catastrophe, for example, explosion, may happen.
Supercapacitors can only store energy but cannot generate energy. Thus, supercapacitors belong to the class of passive device, and two shortcomings can be immediately recognized in the use of supercapacitors. One is the short use-time, and the other is the rapid falling of the capacitor voltages at discharging. Actually, the two defects are all related to the low energy content of supercapacitors. To compensate the handicaps of supercapacitors in power applications, they must work under the support of a power source such as batteries, fuel cells, generators or utility power grid. In the forgoing combination, the unique properties of high power density and fast charging of supercapacitors are fully utilized, and the power level of the power source is significantly amplified. In other words, supercapacitors serve as a load leveling to the aforementioned voltage sources to prolong their lifetime, and to minimize their sizes for the applications.
There are numerous works, particularly in the electric vehicles, using the combination of supercapacitors and batteries as seen in U.S. Pat. No. 5,157,267 issued to Shirata, U.S. Pat. No. 5,373,195 to De Doncker, U.S. Pat. No. 5,642,696 to Matsui, U.S. Pat. No. 5,734,258 to Esser and U.S. Pat. No. 6,617,830 to Nozu, just to name a few. In these prior reports, a plural number of batteries and supercapacitors are grouped into two separate banks, respectively, disposed with electronic circuits containing converters and processors to control the power delivery and recharging of supercapacitors.
Combinatory use of batteries and supercapacitors is also seen in the application of lower power consumption as in U.S. Pat. No. 6,373,152 issued to Wang for power tool. Furthermore, the hybrid of batteries and supercapacitors in conjunction with a switching mechanism for doubling the power output of the hybrid can be found in U.S. Pat. No. 6,016,049 ('049) issued to Baughman and U.S. Pat. No. 6,650,091 ('091) to Shiue. In '049, the battery and supercapacitor are switched from parallel to series connection right before the discharging to a load, whereas only the supercapacitors are switched from parallel to series connection in '091.
All of the prior works using the hybrid power source rely on a bank of batteries for recharging the supercapacitors quickly so that the supercapacitors can provide continuous and stable peak powers. However, the voltages of the supercapacitors fall rapidly at discharging. On the other hand, in many household products driven by disposable or primary alkaline batteries, the end of the battery life does not mean a complete drainage of the energy content of the batteries. As a mater of fact, there is about 65% of energy unused at the time of discarding the batteries because the residual voltages of batteries have fallen below the driving voltages of the products. Therefore, a lot of energy is wasted every time when an alkaline battery is claimed dead.